The invention concerns a cryostat, for the acceptance of a superconducting magnet coil for, nuclear spin resonance (NMR) or ion cyclotron resonance (ICR) to measurements. A central vessel for a cryogenic fluid such as liquid helium, which using holding means preferentially comprising thin-walled suspension tubes, is mechanically connected to, or suspended from, the inner side of a closed outer jacket of the cryostat, whereby the holding means is in good thermal contact with a shell surrounding the central vessel via good thermally conducting tube-shaped connecting elements which completely surround the holding means. The shell is maintained at the temperature of a second cryogenic fluid, such as liquid nitrogen, whose boiling temperature is higher than the boiling temperature of the fluid in the central vessel.
A cryostat of this kind is, by way of example, known from DE 29 06 060 C2.
In constructing a cryostat, thermal contact is often required which exercises as small a mechanical force as possible on the remaining components. For example, it is customary with cryostats for NMR-magnets, to suspend the shell containing the liquid nitrogen using three thin-walled steel tubes equally spaced around the circumference for fastening it to the inside of the outer jacket of the crycstat. Such a suspension is statically defined and leads to no particular problems. In addition, it is necessary to assure that the nitrogen vessel be in thermal contact with both thin-walled steel suspension tubes of the fluid helium containing central vessel in order to achieve better thermal separation of the helium vessel from the temperatures (approx. 300.degree. Kelvin) present in the outer region of the cryostat.
The contact elements between the suspension elements of the helium vessel and its surrounding liquid nitrogen containing shell were, in prior art, normally relatively rigid. In the above mentioned publication, the suspension tubes, which simultaneously serve as venting and filling tubes for the helium vessel, are fastened to connecting tubes by means of rigid aluminium heat transfer collars, the connecting tubes being closed on all sides, thermally conducting, and coaxial to the suspension tubes and, on their other end, are in thermal contact with the liquid nitrogen vessel so that they are largely at the temperature of liquid nitrogen.
This mechanically rigid thermal contacting can lead to a statically undefined mechanical state when cooling the cryostat to the temperature of liquid nitrogen or liquid helium due to the thermal stresses which thereby occur and which must be compensated for by the remaining elasticity of the mechanical components.
As long as the liquid nitrogen containing vessel is symmetrically constructed and the suspension elements (as a rule three) are evenly cooled down, it is possible for the nitrogen vessel to be displaced only in the direction of the suspension elements, that is to say, upward or downward.
Therefore no sideward displacements takes place and, consequently, no disruptions of the available axial symmetry of the crycstat assembly occurs. As a result, both connecting arrangements between the liquid nitrogen vessel and the thermal contacts on both steel tube suspension elements can only transfer forces in a vertical direction of approximately equal magnitude, onto both steel tubes of the helium vessel suspension. The helium vessel can thereby only be displaced vertically by a small amount due to the extremely strong mechanical rigidity of the two steel tubes in this direction. This situation is therefore, in general, not critical.
The situation changes, however, when the three suspension tubes of the nitrogen vessel are not cooled down evenly. In this case the lengths of the three suspension elements change differently and the nitrogen vessel experiences a sideward displacement which, by means of the two connections to the thermal contacts, can be transferred to the helium vessel. The two helium vessel suspension tubes cannot deliver a sufficiently large reaction force against this sideward displacement so that the helium vessel experiences nearly the same sideward displacement as the nitrogen vessel. However, along with the displacement of the helium vessel, this results in a displacement of the center of the cryomagnet coil located therein relative to the stationary measurement sample within the central room temperature bore of the cryostat. The measurement sample thereby experiences a field change which disadvantageously influences the NMR-spectrum and requires at least a new "shimming" of the field. This procedure depends, disadvantageously, on the height of the fluid level of the nitrogen, or more precisely, on the actual weight of this liquid.
The liquid nitrogen vessel suspension tubes cannot normally be cooled down evenly since, for safety reasons, one of the tubes must always, be equipped with an overpressure valve. It is therefore not possible for cold nitrogen gas to boil-off through this tube so that, after some time, it becomes significantly warmer than the other two tubes. In this fashion the above described transverse displacements of the central helium vessel relative to the room temperature bore of the cryostat, normally occur.
Because of the symmetry and the strength of the steel suspension tubes in the vertical direction, those of skill in the art were of the opinion that it is not possible for geometrical displacement to occur while cooling down the cryostat. In contrast, it is the purpose of the present invention to modify a conventional cryostat with as simple means as possible to avoid, during cooling of the cryostat, the above described horizontal displacement of the cryomagnet containing a central vessel due to a tilting of the shell filled with a second cryogenic fluid and surrounding the central vessel, said tilting being transferred to the central vessel.